build,debian: extract reusable Debian format primitives from build/

Move the generic components out of src/build/ into a new src/debian/
module so they can be reused independently of the build pipeline:
deb822 control parsing (plus the debian/control model), file checksum
registry, debian/files registry, Debian version handling and changelog
entry parsing.

Merge OpenPGP clearsigning into utils/gpg.rs next to the existing key
discovery helper, making signing available outside of builds.

Delegate changelog.rs header/footer parsing to the new
debian::changelog parser, removing the duplicate regex implementation.

src/build/ keeps only build-specific logic: the pipeline driver,
build types, environment setup and the .buildinfo/.changes writers.

src/lib.rs

@@ -9,6 +9,9 @@ pub mod apt;
 pub mod build;
 /// Parse or edit a Debian changelog of a source package
 pub mod changelog;
+/// Reusable Debian format primitives (control/deb822, checksums, versions,
+/// changelog entries, artifact registries)
+pub mod debian;
 /// Build a Debian package into a binary (.deb)
 pub mod deb;
 /// Obtain general information about distribution, series, etc